Preheat your oven to 350°F (175°C). Line a 9x9-inch or 9x13-inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later.

In a medium bowl, combine graham cracker crumbs, granulated sugar, salt, and melted butter. Mix with a fork until the texture resembles wet sand.

Press the mixture firmly and evenly into the bottom of the prepared pan using the flat bottom of a measuring cup. Bake for 10 minutes until lightly golden, then set aside to cool slightly while preparing filling.

In a large mixing bowl or stand mixer fitted with paddle attachment, beat the softened cream cheese and sugar on medium speed for 2 minutes until completely smooth and lump-free.

Add eggs one at a time, mixing on low speed just until incorporated. Stir in sour cream, vanilla extract, and lemon juice. Pour the smooth filling evenly over the pre-baked graham crust.

In a separate bowl, toss the finely diced apples with brown sugar, cinnamon, nutmeg, and cornstarch. Spoon and distribute the spiced apples evenly over the cheesecake layer.

In a medium bowl, combine rolled oats, flour, brown sugar, and cinnamon. Add cold cubed butter and cut in using a pastry blender or your fingertips until coarse, pea-sized crumbs form. Scatter streusel evenly over the apples.

Bake at 350°F (175°C) for 35 to 40 minutes, until the streusel is golden brown and the cheesecake center is just set with a slight gentle wobble.

Remove from oven and cool completely at room temperature on a wire rack for 1 hour. Transfer to the refrigerator and chill for at least 3 to 4 hours (or overnight) until firm.

Use parchment paper overhangs to lift bars onto a cutting board. Slice into 16 even squares with a sharp warm knife. Drizzle generously with warm salted caramel sauce and finish with a pinch of flaky sea salt before serving.